PROJECT SUMMARY: Tauopathies are a group of neurodegenerative diseases characterized by aggregates of the Microtubule Associated Protein Tau (tau) found in the brains of patients on autopsy. These disorders include Alzheimer's disease (AD), chronic traumatic encephalopathy (CTE), and frontotemporal dementia (FTD). AD alone is the 6th leading cause of death in the US (affecting more than 5.7 million), no cure is currently available, and the number of patients is expected to substantially increase in the coming years. Tauopathies can be caused by mutations in tau that promote its aggregation (as in FTD) or by extracellular stressors that trigger tau aggregation (as in AD and CTE). How extracellular stressors lead to intracellular tau aggregation is unknown. A number of recent observations indicate that stress granules (SGs), a type of cytosolic RNA and protein assembly that forms in response to cellular stress, might be involved in the formation and persistence of tau aggregates and manipulating SGs could indicate new therapeutic strategies. I propose a model where cellular stressors (e.g., neuroinflammation, viral infection, amyloid-b) promote the formation of SGs that recruit tau. SGs create high local concentrations of tau and RNA that nucleate and stabilize tau aggregates and contribute to their toxicity. This model is supported by in vitro and in vivo evidence suggesting that RNA promotes tau aggregation, RNA is sequestered into tau aggregates in AD patient brains, and that reducing SG formation decreases tau toxicity. My preliminary data shows that stress granules induced by the neuroinflammatory compound, prostaglandin J2, colocalize with tau. I have also shown by electron microscopy and thioflavin T fluorescence that incubating RNA with tau in vitro causes the formation of tau aggregates made up of paired helical filaments that contain RNA. Finally, I have shown by RNAseq and by single molecule fluorescence in situ hybridization (smFISH) that in vivo tau aggregates contain a diversity of RNAs (rRNAs, mRNAs, snRNAs, and lncRNAs) and some RNAs are more enriched than others. Aim 1 of this proposal will identify and manipulate the levels of RNAs that are bound by tau in unaggregated and aggregated states in disease relevant model systems. Successful completion of this aim will suggest 1) what set of RNAs act as cofactors for tau aggregation, 2) whether the RNAs in tau aggregates are similar to SGs, and 3) whether sequestration of specific RNAs contributes to tau toxicity. Aim 2 of this proposal will manipulate SG formation and RNA stability and then measure how these manipulations influence tau aggregation. Results of this aim will determine what role RNA and SGs play in the formation and persistence of tau aggregates. Broadly, this proposal aims to test the hypothesis that SGs link extracellular stress and intracellular tau aggregation with the hope of identifying new therapeutic strategies to treat debilitating neurodegenerative diseases such as AD.